Refrigeration process

ABSTRACT

A refrigeration process employing a single- or plural-stage flashing of a refrigerant liquid to obtain high and/or intermediate temperature cooling, and condensation of refrigerant vapor and subsequent flashing thereof to obtain low temperature cooling, of a heat exchange zone so as to remove heat from said zone. In one embodiment, a gas is passed through said heat exchange zone in heat exchange relationship with said refrigerants and at least partially liquefied.

United States Patent Harper et al.

[ 1 Dec. 30, 1975 REFRIGERATION PROCESS Inventors: Ernest A. Harper; Dunn M. Bailey,

both of Bartlesville, Okla.

Assignee: Phillips Petroleum Company,

Bartlesville, Okla.

Filed: Mar. 4, 1974 Appl. No.: 447,583

Related [1.5. Application Data Division of Ser. No. 75,992, Sept. 28, 1970, Pat. No. 3,808,826.

Primary ExaminerNorman Yudkoff Assistant ExaminerFrank Sever [57] ABSTRACT A refrigeration process employing a singleor plural- [52] US. Cl. 62/9; 62/11; 62/28; Stage flashing of a refrigerant liquid to obtain high 62/40 and/or intermediate temperature cooling, and conden- [51] Int. Cl. F251 l/ sation of refrigerant vapor and subsequent flashing 1 Field of Search 62/9, 11, 40, 28, 23, 24, thereof to obtain low temperature cooling, of a heat 62/27, 29, 30, 40, 31, 37 exchange zone so as to remove heat from said zone. In

- one embodiment, a gas is passed through said heat exl l References Cited change zone in heat exchange relationship with said UNITED STATES PATENTS refrigerants and at least partially liquefied. 2,610,704 9/1951 Patterson 62 /28 1 Claim, 2 Drawing Figures 84 C 118 PIPELINE 6B 7 x 7 P 7 12 a i i E: E i E E E 5 E5: 3: 5: 90 m 1 =2 32 52;; 3i 2 88 I06 62-? 5 5E 35 =2 2 US, Patfint Dec. 30,.1975 Sheet 1 0152 3,929,438

Vmm

REFRIGERATION PROCESS This is a division application of our copending application having Ser. No. 75,992, filed Sept. 28, 1970 now U.S. Pat. No. 3,808,826, filed May 7, 1974.

This invention relates to a refrigeration process.

Various refrigeration processes or cycles are known in the art. In many instances, these refrigeration processes or cycles have been developed for particular purposes. In all instances, it is of the utmost importance to obtain maximum efficiency in the refrigeration cycle so as to keep investment and operating costs at a minimum. This is particularly important, and difficult, when one is cooling and at least partially liquefying a gas such as natural gas.

A typical natural gas, while composed predominantly of methane, will contain significant amounts of higher boiling hydrocarbons. This complicates the problem of cooling and liquefying the natural gas because heat must be removed from the gas over a wide temperature range. Consequently, many of the prior art processes are inefficient and thus uneconomical from the stand point of investment cost and/or operating cost. One prior art method uses the residue gaseous methane stream as the refrigerant. Said residue methane stream is passed through an expansion turbine, thereby lowering its pressure and temperature sufficiently that it can be employed to cool the incoming feed gas stream in countercurrent heat exchangers. in other prior art methods, external refrigeration systems are employed. Depending upon the temperature reduction desired, more than one refrigerant is frequently employed in the well known cascade system. Such a system requires separate compressors for each refrigerant.

The present invention provides a more efficient external refrigeration system which is capable of cooling natural gas to a very low temperature without the need for separate refrigerants and separate compressors. Thus, the refrigerant employed in the practice of the invention can be a single-component refrigerant, for example, an essentially pure hydrocarbon such as ethane or propane, a freon, etc. However, multicomponent refrigerants are preferred for use in the refrigeration system of the invention because they are capable of being employed to effect cooling of material over a wider range of temperature than is possible with singlecomponent refrigerants. This is possible with multicomponent refrigerants because the composition of such refrigerants, as well as the pressure, is changed during the refrigeration cycle in order to obtain the desired refrigeration temperature (boiling point of the refrigerant). One presently preferred refrigerant which can be employed in the practice of the invention comprises a mixture of ethane and propane. However, depending upon the refrigeration temperature desired, it is within the scope of the invention for said refrigerant to contain methane, nitrogen, helium, or other gases more volatile than ethane.

An object of this invention'is to provide an improved refrigeration cycle or process. Another object of this invention is to provide an improved refrigeration cycle or process wherein both a refrigerant vapor and a refrigerant liquid are employed for more efficient refrigeration in the cooling of a heat exchangezone for the removal of heat therefrom. Another object of this inve'ntion is to provide an improved method for cooling and/or liquefying a natural gas. Another object of this invention is to provide an improved method forrecov- 2 ering higher boiling components from a natural gas. Another object of this invention is to provide an improved method for recovering a refrigerant mixture from a natural gas. Other aspects, objects, and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided a method for removing heat from a heat exchange zone and the contents thereof, utilizing refrigerant vapor and refrigerant liquid streams obtained from a compressed and partially liquefied refrigerant, which method comprises: passing a stream of said refrigerant vapor through a first region of said heat exchange zone in indirect heat exchange relationship with at least one stream of said refrigerant liquid having a temperature less than the temperature of said refrigerant vapor so as to cool said vapor; passing said cooled refrigerant vapor through a succeeding downstream region of said heat exchange zone in indirect heat exchange relationship with another stream of refrigerant, obtained as described hereinafter, and at a temperature less than the temperature of said refrigerant liquid, so as to further cool and liquefy said refrigerant vapor; flashing said liquefied refrigerant vapor so as to further cool same and to obtain said another refrigerant; and removing from said heat exchange zone vapors resulting from the vaporization of said refrigerants so as to remove heat from said zone.

Further according to the invention, there is provided a process for recovering a mixture comprising ethane and propane from natural gas, which process comprises, in combination, the steps of: (a) cooling said natural gas under pressure and temperature conditions sufficient to partially liquefy same; (b) fractionating said partially liquefied natural gas from step (a) in a first fractionation zone to recover an overhead stream comprising methane, ethane, and propane, and a bottoms stream comprising some propane and higher boiling hydrocarbons; (c) partially condensing said overhead stream from step (b); (d) returning a first portion of said condensed overhead from step (c) to said first fractionation zone as reflux thereto; (e) passing a second portion of said condensed overhead from step (c) to a second fractionation zone as feed thereto; and (f) in said second fractionation zone, fractionating said feed to recover an overhead stream comprising methane, and a bottoms stream comprising ethane and propane as one product of the process.

Any suitable refrigerant, either a single-component refrigerant or a multicomponent refrigerant, can be employed in the practice of the invention. However, multicomponent refrigerants are usually preferred. One presently preferred refrigerant is a multicomponent refrigerant comprising a mixture of ethane and propane containing from about 5 to about percent ethane, and from about 95 to about 5 percent propane. This refrigerant finds wide application in the recovery of ethane and heavier hydrocarbons from natural gas. Said preferred refrigerant can contain components higher boiling than propane, e..g., butane and/or pentanes, usually present in amounts of from about 0.1 to 5 percent. However, the presence of said higher boiling components is usually not preferred. Depending upon the degree of cooling desired, e.g., whether or not it is desired to completely liquefy a natural gas, said preferred refrigerant can contain from 10 to 50 percent of a component more volatile than ethane, such as methane, helium, nitrogen, etc.

FIG. 1 is a schematic flow sheet diagrammatically illustrating several embodiments of the invention.

FIG. 2 is a schematic flow sheet diagrammatically illustrating another embodiment of the invention.

Referring now to the drawings, wherein like reference numerals have been employed to denote like elements, the invention will be more fully explained. It will be understood that many valves, pumps, control instruments, and other conventional equipment not necessary for explaining the invention have been omitted for the sake of brevity. For convenience, and not by way of limitation, the invention will be described with reference to the partial liquefaction of a stream of natural gas so as to be able to separate ethane and higher molecular weight components from methane and other components more volatile than ethane contained in said natural gas. The ethane and heavier components of natural gas are oftentimes referred to as natural gas liquids. These liquids" include hydrocarbons such as ethane, propane, butanes, pentanes, and sometimes higher molecular weight components, which are valu able as raw materials for preparing various petrochemicals. The more volatile components referred to above include, in addition to methane, such materials as hydrogen, nitrogen, helium, and the like. The natural gas feed stream may sometimes be obtained as the effluent from a natural gasoline plant and will have had at least a portion of the heavier hydrocarbons, water, and carbon dioxide removed. In the calculated example used hereinafter in connection with the description of the drawings, the natural gas stream has been dehydrated to a -l F. dew point by conventional means not shown, and contains less than 0.02 mol percent carbon dioxide. While not shown in the drawings, it will be understood that if a wet gas stream is to be refrigerated for the liquefaction thereof, and/or the recovery of heavy hydrocarbons therefrom, provision should be made to dehydrate the gas for water removal and to withdraw compounds such as benzene and carbon dioxide which would solidify at the low temperatures employed. Such materials in liquid state can be tapped off from the heat exchangers at appropriate points of temperature and pressure. It should also be understood that the various operating conditions given herein in connection with the description of the drawing are not to be construed as limiting on the invention. Said conditions can be varied depending upon the gas or other material being cooled, the amount of cooling desired, and in the case of fractionators the separation to be effected.

The drawings will be described with reference to a calculated example using a typical natural gas having the composition:

In one embodiment of the invention, a natural gas stream at a temperature of about 90 F. and a pressure of about 550 psig is passed via conduit 10 into heat exchanger 12 wherein it is cooled to a temperature of about F. by indirect heat exchange with the cold refrigerants and employing the refrigeration cycle of the invention as described hereinafter. Said heat exchanger 12 can be any suitable type of heat exchanger. As here illustrated diagrammatically, said heat exchanger comprises a plate-type exchange wherein the various segments of the exchanger are all housed in one shell. Such exchangers are conventional and well known in the art. However, it is within the scope of the invention to employ three or more individual exchangers instead of a unitary exchanger as illustrated. It is also within the scope of the invention to employ tube and shell-type exchangers instead of a plate-type ex changer.

The reduction in temperature of said natural gas stream during its passage through heat exchanger 12 causes about 20 mol percent of the gas to liquefy. About 50 percent of the ethane, about percent of the propane, and all of the heavier hydrocarbons will be liquefied under the above-described conditions. The partially liquefied stream is withdrawn from heat exchanger 12 via conduit 14 and passed into phase separator 16. The noncondensed gas comprising methane is returned to said exchanger 12 via conduit 18 for countercurrent heat exchange with the incoming gas stream in conduit 10. The now warmed residue gas exits from said heat exchanger via conduit 20 for delivery to a pipeline or other use. If desired, said residue gas can be withdrawn from the system via conduit 22 instead of being returned to said heat exchanger 12. If desired, the liquefied hydrocarbons in separator 16 can be withdrawn therefrom via conduit 24. Preferably, said liquefied hydrocarbons are passed through expansion valve 26 for flash vaporization of a portion thereof by reducing the pressure to about 410 psig and the temperature to about 8l F. The resulting mixture of liquid and vapor is passed into phase separator 28 wherein the two phases are separated as indicated in the drawing. Said two phases are withdrawn from separator 28 separately and, preferably, are recombined prior to being returned to heat exchanger 12 for passage through at least a portion thereof. The use of a separator such as separator 28 is preferred in order to distribute the liquid phase uniformly with the gas phase when two phases are being passed through the heat exchanger. Here, and elsewhere, where a liquid and vapor are both introduced into a heat exchanger it may be desirable to employ a liquid-vapor distributor such as described in U.S. Pat. No. 3,158,010, issued Nov. 24, 1964.

The partially warmed, recombined stream is withdrawn from heat exchanger 12 via conduit 30 and passed into deethanizer 32. A stream comprising essentially all the methane and ethane is removed overhead from said deethanizer and, preferably, is returned to said heat exchanger 12 via conduit 34 for additional heat exchange with the incoming gas stream in conduit 10. Said residue gas in conduit 34 can be removed from exchanger 12 via conduit 36, recompressed to pipeline pressure by compressor 38, and combined with the residue gas in conduit 20. If desired, said residue gas can be removed from the system through conduit 40 prior to compressor 38 or through conduit 42 after compressor 38, instead of passing same into the pipeline. The bottoms from deethanizer 32 comprises propane and heavier hydrocarbons and are recovered as one product of the system via conduit 44.

In one presently preferred refrigeration cycle employed to refrigerate the natural gas stream just described, a multicomponent refrigerant comprising a mixture of ethane and propane is advantageously employed. In said preferred refrigeration cycle, there is employed two-stage flashing of a refrigerant liquid to obtain high and intermediate temperature cooling and single-stage condensation of refrigerant vapor to obtain low temperature cooling. Said mixture of ethane and propane is compressed in the compressor system illustrated to a pressure of about 220 psig, cooled to a temperature of about 95 F. in water cooled exchanger 46 and passed via conduit 48 into surge or storage tank 50 wherein it separates into a liquid phase and a gaseous phase. The liquid phase contains about 13 mol percent ethane, the remainder being propane. The vapor phase contains about 31 mol percent ethane, with the remainder being propane. Thus, two refrigerant streams are made available by partial liquefaction of a single, compressed refrigerant stream. The two refrigerant streams have different boiling points (at the same pressure) by virtue of having different compositions and therefore are advantageously used at different temperature levels in the refrigeration cycle.

Refrigerant vapor is withdrawn via conduit 52 and introduced into heat exchanger 12 wherein it is cooled to a temperature of about 75 F. and completely liquefied by heat exchange with colder refrigerant streams as described hereinafter.

Refrigerant liquid is withdrawn from storage or surge tank 50 via conduit 54, passed through expansion valve 56 wherein it is expanded or flashed to form a liquid and vapor, and introduced into separator 58 for phase separation. In passing through said expansion valve 56 the pressure on said refrigerant liquid is reduced to about 91 psig and the temperature is reduced to about 45 F.

Refrigerant liquid is withdrawn from separator 58 via conduit 60 and a portion thereof is passed via conduit 62 into a first region (b) of said heat exchanger 12 wherein it is passed in indirect heat exchange relationship with said stream of refrigerant vapor introduced via conduit 52 and said natural gas introduced via conduit 10. The resulting vaporized refrigerant is withdrawn from heat exchanger 12 via conduit 64, passed through conduit 65 to knockout drum 66, compressed in the high stage 68 of the three-stage compressor system illustrated, then through cooler 46 wherein it is partially liquefied, and then passed via conduit 48 into surge or storage tank 50, previously described. Refrigerant vapor in separator 58 is withdrawn therefrom via conduit 70 and introduced into conduit 65 for passage through said high stage compression zone 68, previously described, to complete the cycle for both the liquid and vapor from separator 58. It is desirable to maintain a substantially constant ratio between the refrigerant vapors flowing in conduits 64 and 52 because the refrigerant in conduit 64 (vaporized refrigerant from conduit 62) contributes materially to the cooling and condensation of the refrigerant vapor in conduit 52. Thus, should the flow in conduit 52 increase, the flow in conduit 64 should be proportionally increased. The desired ratio control of said two streams can be conveniently accomplished by employing ratio controller 74 to adjust or regulate valve 72 in conduit 70 responsive to the flow in said conduits 52 and 64. For example, if the flow in conduit 64 drops below a specified ratio of that in conduit 52, said valve 72 will 0 be closed somewhat, thus increasing the pressure in conduit 62 with resulting increase in flow in conduit 64 until the desired ratio is reestablished.

A second portion or stream of said refrigerant liquid from separator 58 is passed from conduit 60 through expansion valve 76 wherein it is expanded or flashed to reduce its pressure to about 24 psig and its temperature to about 7 F., and then passed into phase separator 78 wherein a phase separation of the partially vaporized refrigerant is effected. Liquid refrigerant in separator 78 is withdrawn therefrom via conduit 80 and introduced into a second region (0) of said heat exchanger 12, downstream from said first region (b) with respect to the flow of said refrigerant vapor through conduit 52. Said refrigerant liquid from conduit 80 is passed in indirect heat exchange relationship in exchanger 12 with said stream of refrigerant vapor in conduit 52 and said stream of gas introduced via conduit 10 so as to vaporize said liquid refrigerant and further cool said stream of refrigerant vapor and said stream of gas. Vapors resulting from the vaporization of the refrigerant in conduit 80 are withdrawn from said heat exchanger 12 via conduit 82 and passed via conduit 84 and knockout drum 86 into the intermediate stage 88 of the compressor system shown. Effluent from compression stage 88 is passed through cooler 90 for partial cooling and then combined with the refriger ant vapors in conduit 65 for further compression therewith in compression stage 68 and return to said storage zone or surge tank 50 as previously described. Refrigerant vapor in separator 78 is withdrawn therefrom via conduit 92 and combined in conduit 84 with the vapors from conduit 82 for compression, as previously described, to complete the cycle for both the liquid and vapor from separator 58.

Returning now to the stream of refrigerant vapor introduced into heat exchanger 12 via conduit 52, said stream is completely liquefied during its passage through heat exchanger 12, is withdrawn from said heat exchanger via conduit 94, passed through expansion valve 96 wherein its pressure is reduced to about 4 psig and its temperature is reduced to about 80 F., and then introduced into phase separator 98 wherein a phase separation between the two phases is effected. The liquid and vapor phases in separator 98 are preferably withdrawn therefrom separately and recombined as shown, and then introduced into said heat exchanger 12 via conduit 100 for passage through a third region (d) of said heat exchanger, downstream from said second region (0) with respect to the flow of said refrigerant vapor in conduit 52 and the natural gas in conduit 10, and passing said refrigerant in indirect heat exchange relationship with said streams in conduits 52 and 10 so as to vaporize said refrigerant in conduit 100, further cool and liquefy the refrigerant gas in conduit 52, and further cool and partially liquefy the natural gas in said conduit 10. Refrigerant vapor is withdrawn withdrawn from segment or region (d) of said heat exchanger 12 via conduit 102, passed through knockout drum 104, and compressed in low stage 106 of the compressor system shown. The effluent from low stage compression 106 is combined with the feed to intermediate stage compression 88 for further compression, cooling, and returned to said storage tank or surge tank 50, as previously described to complete the cycle.

Itis important that the refrigerant mixture in separator 98 be at the lowest temperature in the system, for example about 80 F., in order to cool the gas stream in conduit 10 to about 75 F. In order to control the refrigerant temperature in said separator 98 at said desired 80 F., said expansion valve 96 is adjusted or regulated by flow recorder controller 108 responsive to the gas flow in conduit 52 as measured by flow meter 110 and transmitted via transmitter 112. The set point of said flow recorder controller 108 is in turn reset or regulated by temperature recorder controller 114 re sponsive to the measured temperature of the stream in conduit 14 as transmitted by transmitter 116. Thus, the desired temperature of the refrigerant in separator 16 is obtained by applying that temperature as the set point to temperature recorder controller 114.

The above-described refrigeration cycle employing two-stage flashing of a refrigerant liquid to obtain high and intermediate temperature cooling, and condensation of refrigerant vapor with subsequent flashing thereof to obtain low temperature cooling, represents one presently preferred embodiment of the invention. However, it is within the scope of the invention to employ only one-stage flashing, or three-, four-, or five-stage flashing, or more, of said refrigerant liquid. Thus, in some situations where a gas feed stream, or other material, is to be cooled over a more narrow temperature range, the liquid refrigerant from storage or surge tank 50 is flashed only once. For example, only the first stage flashing through expansion valve 56 is used, and the second stage flashing through expansion valve 76 is omitted. In such instances, heat exchange zone 12 would comprise only two regions, e.g., regions (b) and (d) (extended if desired or necessary to cover the entire zone), and region (c) would be omitted. Appropriate changes in the compression portion of the cycle can be made in view of the disclosure herein.

In other instances where it is desired to cool a gas stream or other material over a more extended temperature range, the liquid refrigerant from storage or surge tank 50 can be serially flashed three or more times to provide refrigeration at three or more temperature levels prior to the lowest temperature level, e.g., region (d). In such instances, heat exchange zone 12 would comprise more than three regions, e.g., regions (b), (-1), (0-2), and (d), etc. Again, appropriate changes can be made in the compression portion of the cycle.

In still other embodiments of the invention, the refrigerant vapors resulting from the liquid refrigerant introduced into segment or region (c) of heat exchanger 12 via conduitfitl, instead of being withdrawn from said heat exchanger via conduit 82, can be withdrawn from said heat exchanger via conduit 118 and introduced into conduit 84 for compression and further handling to complete the cycle as previously described. Similarly, if desired, vapors resulting from the refrigerant introduced into said heat exchanger 12 via conduit 100, instead of being withdrawn from said heat exchanger via conduit 102, can be withdrawn from said heat exchanger via conduit 120 and then introduced into conduit 102 for compression and further handling to complete the cycle as previously described.

In still another embodiment of the invention, the refrigerant liquid withdrawn from storage or surge tank 50 via conduit 54 is divided into two portions. A first portion is passed through expansion valve 56 into separator 58. The refrigerant liquid in separator 58 is withdrawn therefrom via conduit 60 and passed via conduit 62 into segment or region (b) of heat exchanger 12, as previously described. The second portion of the refrigerant liquid in conduit 54 is passed via conduit 55,

8 expansion valve 57 wherein it is expanded or flashed and its temperature reduced to about 50 F. and its pressure reduced to about psig, and then introduced into phase separator 59. Refrigerant vapor from separator 59 is withdrawn therefrom and passed via conduit 61 into conduit 70 for further handling as previously described. Refrigerant liquid from said separator 59 is passed via conduit 63, expansion valve 76 wherein its pressure is lowered to about 24 psig and its temperature is lowered to about -7 F., and then introduced into phase separator 78. The refrigerant vapor and refrigerant liquid in said phase separator 78 are handled as previously described.

Make-up refrigerant can be introduced into the system via conduit downstream from expansion valve 56.

It is a feature of the invention that a single refrigerant of multicomponent composition is employed at a plurality of temperature or pressure levels for efficient cooling of a heat exchange zone and materials passed through said heat exchange zone. Thus, in a presently preferred embodiment, a refrigerant liquid is employed at its highest refrigerating temperature in segment or region (b) of heat exchanger 12 where, because of its higher pressure and highest propane content (about 91 percent), it effects the first cooling of the warm incoming streams in said conduits I0 and 52. To obtain a lower temperature refrigerant for additional cooling of said incoming streams, a second stream of the refrigerant liquid employed in region (b) of said heat exchanger 12 is expanded or flashed through expansion valve 76 to thereby obtain a lower boiling mixture, which by virtue of its reduced pressure is then employed in said second region or section (c) of said heat exchanger 12. In order to obtain a still lower temperature refrigerant for use in cooling said incoming streams to their lowest desired temperature, the gaseous refrigerant from storage tank or surge tank 50 is employed which, because of its increased ethane content (about 31 percent) boils at the lowest temperature after liquefaction by passage through said heat exchanger 12 and flashing or expanding same through said expansion valve 96.

Said presently preferred embodiment of the invention thus provides three temperature levels of refrigeration by employing two-stage flashing of a refrigerant liquid to obtain the high temperature and intermediate temperature levels of cooling, and employing a singlestage condensation of refrigerant vapor, which refrigerant vapor after liquefaction is expanded to obtain the lowest stage of cooling.

Referring now FIG. 2, there is illustrated another embodiment of the invention which can be employed for recovering a mixture of ethane and propane from a mixture of methane, ethane, and heavier hydrocarbons extracted from a natural gas stream by cooling and partially liquefying said natural gas stream. Said mixture of ethane propane can conveniently be employed as the multicomponent refrigerant employed in the refrigeration system illustrated in FIG. 1.

A mixture of partially liquefied hydrocarbons comprising methane, ethane, and heavier hydrocarbons, having been recovered from a natural gas stream as described in connection with FIG. 1, is passed via conduit 122 into a first fractionation zone or deethanizer 124 as feedstock thereto. In said deethanizer, said feedstock is fractionated to recover an overhead stream comprising methane, ethane, and some propane. Typical operating conditions 'infractionator 124 would include a top tower temperature of about"-ll"F., a bottom tower temperature of about 2l9-F;,' and a pressure of 412 psig. Said overhead stream "is removed via conduit 126, partially condensed in condenser 128, and passed into accumulator 130. A vapor stream com prising methane is removed from said accumulator 130 via conduit 132. A first portion of the condensed over head is removed from accumulator 130 and passed via conduit 134 into saidfractionator 124 near the top thereof, as reflux liquid. Preferably, the amount of said reflux passed via conduit 134 is set at a predetermined amount and controlled by rate of flow controller 136. A second portion of said condensed overhead is removed from said accumulator 130 and passed via conduit 138 to a second fractionator or demethanizer 140 as feedstock thereto. Typical operating conditions in fractionator 140 would include a top tower temperature of about -l6 F., a bottom tower temperature of about 46 F., and a pressure of about 412 psig. An overhead stream is withdrawn from fractionator 140 via conduit 142, passed into conduit 126 wherein it is combined with the overhead from said first fractionator 124, and the total stream passed through said cooler 128 wherein it is cooled to a temperature of about -35 F. and thereby partially condensed or liquefied. Any suitable refrigerant can be employed in said cooler 128. if the embodiment of the invention illustrated in FIG. 2 is being employed in combination with the embodiment of the invention illustrated in FIG. 1, said refrigerant can conveniently be a portion of the refrigerant in conduit 80 in said FIG. I. Said accumulator 130 thus serves as a common accumulator for both said fractionator 124 and said fractionator 140. Thus, the same liquid is used as reflux in fractionator 124 and as feedstock in fractionator 140. Preferably, the amount of said feedstock passed to fractionator 140 is controlled in accordance with the liquid level in reboiler 144. Flow of said feedstock can be regulated by means of valve 146 responsive to a signal from liquid level controller 148. A product mixture comprising ethane and propane is withdrawn from reboiler region 144 of fractionator 140 by means of conduit 150. If desired, the rate of withdrawal of said product stream can be controlled by means of a rate of flow controller, not shown. A second product of the process is withdrawn as bottoms product from fractionator 124, being withdrawn from reboiler region 152 via conduit 154. The rate of withdrawal of said second product can be controlled by valve 156 responsive to a signal from liquid level controller 158.

Said fractionator 124 can be conveniently reboiled by means of a hot heat exchange medium supplied by pipe or conduit 160 to reboiler region 152. The flow rate of said hot heat exchange medium can be controlled by valve 162 which can be adjusted or regulated by temperature recorder controller 164 responsive to a temperature measurement in the lower portion of fractionator 124, which is transmitted by transmitter 166. The set point of temperature recorder controller 164 can be in turn regulated responsive to a signal from liquid level controller 168 on phase separator 130. Thus, said fractionator 124 will be reboiled sufficiently to maintain a desired liquid level in phase separator 130, Le, to maintain sufficient liquid to supply reflux to said fractionator 124 and feedstock to said fractionator 140.

Said fractionator can be advantageouslyreboiled by passing a portion of the warm bottoms stream (about 218 1 from reboiler region 152 via'conduits 154 with the flow rate thereof being controlled by valve 172 which is regulated by temperature recordercontroller 174 responsive to the measured temeprature (about 46 F.) in reboiler region 144. Effluent heating fluid from reboiler region 144 is returned to conduit 154 via conduit 176 and removed from-the system with the product-in saidconduit 154. The ethane-propane refrigerant mixture recovered as product via conduit will typically comprise about 60 percent ethane and about 40 percent propane. However, it is within the scope of the invention to recover mixtures having different proportions of said components. For example, the composition of said refrigerant mixture can vary from about 25 to about 95 mol percent ethane and from about 75 to about 5 percent propane.

The embodiment of the invention illustrated in FIG. 2 thus provides a process for recovering a desired mixture of ethane and propane from the overhead of deethanizer in a cryogenic natural gas processing plant. The recovery of said refrigerant mixture is effected in a demethanizing fractionator which is refluxed and fed with part of the condensed overhead stream from said deethanizer. Further, the demethanizing fractionator producing the desired refrigerant mixture is reboiled with bottoms from said deethanizer, thereby reboiling costs. Thus, the fractionation method by which said refrigerant mixture is produced requires neither separate reflux facilities nor external heat for reboiling.

While the invention has been described with particular reference to the cooling of a natural gas in a heat exchange zone, the invention is not limited thereto. The refrigeration system of the invention can be employed to cool other materials contained in said zone. For example, the three regions of the heat exchange zone could comprise three refrigeration zones for storing different materials at three different temperatures, e.g., three different refrigeration rooms in a large cold storage plant.

While certain embodiments of the invention have been described for illustrative purposes, the invention is not limited thereto. Various other modifications or embodiments of the invention will be apparent to those skilled in the art in view of this disclosure. Such modifications or embodiments are within the spirit and scope of the disclosure.

We claim:

1. A process for recovering a mixture comprising ethane and propane from natural gas, which process comprises, in combination, the steps of:

a. cooling said natural gas partially to liquefy said natural gas;

b. distilling said partially liquefied natural gas from step (a) in a first distillation zone to recover an overhead stream comprising methane, ethane, and propane and to recover from a first reboiler zone of said first distillation zone a bottoms stream comprising some propane and higher boiling hydrocarbons;

c. partially condensing said overhead stream from step (b);

d. in a second distillation zone, distilling a feed, as

obtained below, to recover from said second distillation zone an overhead steam comprising methane which overhead stream is then combined with said h. recovering a bottoms steam comprising ethane and propane, as a product of the process, from a second reboiler zone in said second distillation zone, said second reboiler zone heated by indirect heat exchange with liquid from said first reboiler zone said liquid flow controlled in accordance with the temperature in said second reboiler zone; and

i. controlling the amount of heat supplied to said first reboiler zone in accordance with the level in the common accumulator zone so as to maintain a desired level in said common accumulator zone.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION Q PATENT NO. 3 929 43 DATED 2 December 30, 1975 |NVENTOR(5) 3 Ernest A. Harper et al It is certified that error appears in the above-identified patent and that said Letters Patent Q are hereby corrected as shown below:

Column 11, line 3, "steam" should be stream Column 12, line 1, "steam" should be stream gigncd and Scaled this thirteenth Day of April1976 g [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN 0 Alrz'sring Officer Commissionernflarvhrs and Trademarks a x I 

1. A PROCESS FOR RECOVERING A MIXTURE COMPRISING ETHANE AND PROPANE FROM NATURAL GAS, WHICH PROCESS COMPRISES, IN COMBINATION, THE STEPS OF: A. COOLING SAID NATURAL GAS PARTIALLY TO LIQUEFY SAID NATURAL GAS; B. DISTILLING SAID PARTIALLY LIQUEFIED GAS FROM STEP (A) IN A FIRST DISTILLATION ZONE TO RECOVER AN OVERHEAD STREAM COMPRISING METHANE, ETHANE, AND PROPANE, AND TO RECOVER FROM A FIRST REBOILER ZONE OF SAID FIRST DISTILLATION ZONE A BOTTOMS STREAM COMPRISING SOME PROPANE AND HIGHER BOILING HYDROCARBONS; C. PARTIALLY CONDENSING SAID OVERHEAD STREAM FROM STEP (B); D. IN A SECOND DISTILLATION ZONE, DISTILLING A FEED, AS OBTAINED BELOW, TO RECOVER FROM SAID SECOND DISTILLATION ZONE AN OVERHEAD STREAM COMPRISING METHANE WHICH OVERHEAD STREAM IS THEN COMBINED WITH SAID OVERHEAD STREAM FROM STEP (B) AND PASSED TO A COMMON ACCUMULATOR TO FORM A MIXTURE; E. REMOVING A STEAM COMPRISING METHANE FROM SAID COMMON ACCUMULATOR ZONE; F. RETURNING A FIRST PORTION OF SAID ACCUMULATOR MIXTURE TO SAID FIRST DISTILLATION ZONE AS REFLUX TO THE FIRST DISTALLATION ZONE; G. PASSING A SECOND PORTION OF SID ACCUMULATOR MIXTURE CONTROLLED IN ACCORDANCE WITH THE LIQUID LEVEL IN SAID REBOILER REGION OF THE SECOND DISTILLATION ZONE TO THE SECOND DISTILLATION ZONE AS FEED TO SAID SECOND DISTILLATION ZONE; H. RECOVERING A BOTTOMS STEAM COMPRISING ETHANE AND PROPANE, AS A PRODUCT OF THE PROCESS, FROM A SECOND REBOILER ZONE IN SAID SECOND DISTILLATION ZONE, SAID SECOND REBOILER ZONE HEATED BY INDIRECT HEAT EXCHANGE WITH LIQUID FROM SAID FIRST REBOILER ZONE SAID LIQUID FLOW CONTROLLED IN ACCORDANCE WITH THE TEMPERATURE IN SAID SECOND REBOILER ZONE; AND I. CONTROLLING THE AMOUNT OF HEAT SUPPLIED TO SAID FIRST REBOILER ZONE IN ACCORDANCE WITH THE LEVEL IN THE COMMON ACCUMULATOR ZONE SO AS TO MAINTAIN A DESIRED LEVEL IN SAID COMMON ACCUMULATOR ZONE. 